1. Field of the Invention
This invention relates to a polarization separating element in use of a kind of optical device and a manufacturing method thereof.
2. Related Art
A light pickup device used for recording and reproduction of optical or optomagnetic recording disks such as CD, DVD and MO (hereinafter simply referred to as optical disks) includes one shown in FIG. 5A. In the light pickup device 2A of FIG. 5 laser light LA emitted from a laser light source 21 passes through a polarization separating element 1A, a quarter-wave plate 22 and an objective lens 23 and is condensed on the recording surface 241 of an optical disk 24, and reflected light LB again passes through the objective lens 23, the quarter-wave plate 22 and the polarization separating element 1A, and reaches an receptor 25.
The polarization separating element 1A used in the light pickup device 2A is generally designed to diffract all the light rays except either ordinary rays or extraordinary rays so as to obtain greater utilization of laser power. Such diffraction characteristics will hereinafter be referred to as perfectly polarizing properties. For example, where the polarization separating element 1A is designed not to diffract ordinary rays but extraordinary rays, the laser light LA from the laser light source 21 is not diffracted as ordinary light. On the other hand, the reflected light LB from the optical disk 24 has been converted to extraordinary light while passing through the quarter-wave plate 22 by the time when it again enters the polarization separating element 1A. Therefore the light LB is diffracted by the polarization separating element 1A and reaches the receptor 25 as diffracted light of first order LB1. Where such a polarization separating element 1A having perfectly polarizing properties (hereinafter referred to as perfect polarization separating element) is used, all the laser rays LA emitted from the laser light source 21 are condensed on the recording surface 241 of the optical disk 24, and the theoretical maximum of the diffracted light of first order of the reflected light LB reaches the receptor 25, thereby to enjoy greater efficiency of laser power.
However, there are cases in which a substrate 242 of the optical disk 24, through which light should pass before reaching the recording surface 242, exhibits large birefringence. Such an optical disk involves a fear that the reflected light LB may not reach the receptor 25. For example, if the substrate 242 of the optical disk 24 acts like a halfwave plate on account of its birefringence, both the laser light LA emitted from the laser light source 21 and the reflected light LB are ordinary rays. In this case, as shown in FIG. 5B, the reflected light LB passes through the polarization separating element 1A without being diffracted and fails to reach the receptor 25.
The inventors of the present invention now propose use of a polarization separating element which is capable of generating diffracted light rays from both ordinary rays and extraordinary rays in a light pickup device in place of the customarily used perfect polarization separating element 1A. Such diffraction characteristics of the polarization separating element of the invention will hereinafter be referred to as partially polarizing properties.
A polarization separating element having partially polarizing properties (hereinafter referred to as partial polarization separating element) can be prepared in the same manner as for perfect polarization separating element by using lithium niobate. For example, a single crystal of lithium niobate is subjected to periodic proton exchange to form a periodic grating, and an optically isotropic transparent film of SiO.sub.2, etc. is formed on the proton-exchanged area of the periodic grating to obtain a partial polarization separating element. However, a partial polarization separating element obtained from lithium niobate in a conventional process is very expensive for the following reasons. (1) A special apparatus is required for forming a single crystal of lithium niobate, and production of a polarization separating element from the single crystal involves extremely laborious steps. (2) Lithium niobate is brittle and easily broken during handling, resulting in poor yield. (3) The material cost of lithium niobate itself is high.